1. Field of the Invention
The invention relates to a driver circuit, more particularly to a driver circuit for an attachment unit interface used in a network system.
2. Description of the Related Art
In IEEE 802.3 Ethernet applications, a data terminal equipment (DTE) (1) is connected to a media attachment unit (MAU) (5) through an attachment unit interface (AUI) (3), as best illustrated in FIG. 1 The AUI (3) carries encoded control and data signals between the physical layer signaling sub-layer of the DTE (1) and the physical medium attachment sub-layer of the MAU (5) and provides for duplex encoded data and control signal transmission. The AUI (3) often serves as a drop cable between the DTE (1) and the transceiver on the Ethernet tap. All signals in the AUI cable are differential. The AUI signals are driven by conventional differential drivers capable of driving the specified 78 .OMEGA. cable. A detailed description of the electrical characteristics of the AUI signals can be found in the IEEE 802.3 specification. The major characteristics of the AUI signals are summarized briefly in FIGS. 2 and 3. FIG. 2 is a graph which illustrates the amplitude requirement of the transmitted signals in the AUI cable. The maximum rise and fall times is 5 nsec, and the maximum signal overshoot is V1. The ringing of the signals is limited between V2 and V3.
FIG. 3 is a graph illustrating the signal requirement for the AUI driver when the driver ends its active state and enters into its idle state. The AUI driver should be able to maintain a high level output for at least 2 bit times (one bit time is 100 nsec) after the last low-to-high signal transition. The AUI driver should then transit to its idle state with a maximum 40 mV offset voltage within 80 bit times. The undershoot during the transition to the idle state, if any, should be less than 100 mV.
The conventional differential driver circuits that are used to drive the AUI signals are usually implemented in emitter coupled logic (ECL) circuitry. Some of the drawbacks of conventional ECL drivers include difficulty in providing a low idle offset output voltage and the generation of a relatively large undershoot during the active-to-idle transition especially when an inductive load, such as the AUI cable, is driven. Furthermore, conventional ECL drivers are complicated and suffer from large power consumption.